Method of electrolessly depositing metals on particles



United States Patent Ofice 3,511,683 Patented May 12, 1970 3,511,683METHOD OF ELECTROLESSLY DEPOSITING METALS N PARTICLES Wilton F.Espenscheid and Israel J. Heilwell, Princeton,

N..I., assignors to Mobil Oil Corporation, a corporation of New York NoDrawing. Filed June 20, 1967, Ser. No. 647,345 Int. Cl. C23c 3/02 U.S.Cl. 11747 24 Claims ABSTRACT OF THE DISCLOSURE This specificationdiscloses a method for depositing a metal on a high surface areacolloidal substrate. The substrate particles may consist entirely of, ormay be a nonmetal having a surface coating of, a metal which is, in theelectromotive series, above the metal to be deposited on them. Themethod involves dissolving a compound on the metal to be deposited onthe substrate in a nonaqueous anhydrous solvent and adding the substrateto the resulting solution. In a specific embodiment, there is firstdeposited electrolessly on the substrate particles a reactive metalwhich is, in the electromotive series, above the metal to be depositedon them from the nonaqueous anhydrous solvent.

CROSS-REFERENCE TO RELATED APPLICATION The operation of electrolessdeposition, as described herein, is also disclosed in copendingapplication, Ser. No. 647,344, filed June 20, 1967, and certain productsof the method of said application are useful as substrates for thepresent invention.

BACKGROUND OF THE INVENTION (1) The field of the invention comprisesmetal deposition, including a method for the same and the resultingproduct.

(2) So far as is known, the deposition of metals like platinum, eitherelectrolessly or electrolytically, on a substrate comprising particlesin the colloid size range, is not described in the literature, patent orotherwise. The method disclosed herein, comprising the immersiondeposition of these metals in a nonaqueous solution, provides a novelmeans of obtaining colloidal particles coated with the metals.

SUMMARY OF THE INVENTION Platinum and similar metals are deposited oncolloidal particles composed entirely or partly of metal by immersiondeposition in nonaqueous solutions. The substrate particles have outersurfaces comprising a reactive metal which is above platinum in the EMFseries, and on immersion of the same in a nonaqueous solution of asuitable platinum compound, they become coated by replacement of thereactive metal by platinum.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS The invention is of particularvalue in providing for the deposition of platinum on high surface areacolloidal particles for, as indicated, the published literature does notdisclose a way which accomplishes this result. The product iscontemplated as having considerable importance in, among otherapplications, fuel cell and zinc-air battery technologies by permittinga decrease in platinum concentration presently required for airelectrodes. In the field of catalysis, the avoidance of the aqueousphase in the preparation of the metal-coated particles enables activemetals to be deposited, such as the alkali and alkaline earth metals,and at the same time decrease the effects of any adverse surfacechemistry of the substrate or of the deposit itself during deposition ofmetal thereon.

Platinum is one of a group of metals that either cannot be deposited onhigh surface area substrate particles, or can be deposited only withdifiiculty. Other metals of this group include palladium, ruthenium,rhodium, iridium, and osmium, which together with platinum are sometimesreferred to as the platinum series of metals. Also included are GroupVIII metals generally. Although depositable with more ease, metals likesilver, gold, and Group I-B metals are useful in the invention. Forthese, the invention offers a way of depositing them on the describedsubstrates. Other suitable metals are the alkali and alkaline earthmetals which are too active to be deposited from aqueous solutions. Alsocontemplated are the less active metals like titanium, molybdenum,zirconium, tungsten, lead, tin, hafnium vanadium and germanium, which,although thermodynamically capable of depositing from aqueous solutions,do not do so because of solvation effects and the nonreactivity of theirions. In general, the depositable metals are those which form compoundssoluble in the nonaqueous solvent and which, in the EMF series ofmetals, lie below the reactive metal of which the surfaces of thesubstrate are composed. Preferred metals to be deposited are platinum,and the other members of the platinum series. It should be understoodthat more than one metal may be deposited; thus two or more metals maybe deposited together or one after another to form layered deposits.

Compounds of the metal to be deposited include inorganic salts likeplatinum tetrachloride, palladium dichloride, silver perchlorate, ferricchloride, stannic chloride, stannous chloride, gold chloride, etc. Alsosuitable are organometallic compounds conventionally used as petroleumadditives, comprising metal salts of alkyl, aryl, and alkyl-aryldithiophoesphates, sulfonates, sulfates, carboxylates, phosphonates,phosphates, phenates, etc., the metal moiety of which is a metal of thegroups described. Other useful metal compounds are coordinated metalderivatives of olefins, acetylenes, and aromatics, as may be illustratedby such compounds as octadiene complexes of gold; silver acetylide;bis-acrylonitrile-metal derivatives; tris (pi-allyl)iridium;(arene)tungsten(CO) complexes where arene may be benzene, toluene,p-xylene, mesitylene, etc. The metal compound may be used alone or inadmixture with one or more other such compounds. It will be understoodthat the metal compound selected from the foregoing is soluble in thenon-aqueous solvents contemplated herein.

The useful non-aqueous solvents for dissolving the metal compoundsinclude both cyclic and aliphatic compounds. The cyclics comprisearomatics like benzene, toluene, the xylenes, ethylbenzene,propylbenzene, the trimethylbenzenes, cymene, etc.; cycloalkanes likecyclohexane, methylcyclohexane, cyclopentane, etc.; heterocyclics likepyridine, furan, the picolines, etc. Aliphatics preferably include suchpolar compounds as hydrazine; the alkyl formamides; alkyl sulfones andsulfoxides; alkyl halides; and various organic derivatives of carbonicacid like ethylene or propylene carbonates, etc. These solvents, whichare normally liquid, may be used singly or in combinations of two ormore. Also useful are liquefied sulfur dioxide, ammonia, hydrogensulfide, phosgene, etc. The presence of conventional auxiliary agents inthe solution is frequently helpful, such as a mixture of acetic acid andacetic acid anhydride, or the latter alone, which helps to maintainanhydrous conditions and to insure uniform adhering films of depositedmetal. Other useful conventional auxiliary agents or modifiers forpromoting deposition are phosphorodithioic acid; alkyl and aryl sulfonicacids and mixed alkyl-aryl sulfonic acids; and ethylenediamine andderivatives.

The high surface area substrate particles may, as noted, be entirely orpartly of metal; in any event they have outer surfaces composed of areactive metal which, in the EMF series, is above the metal of the metalcompound dissolved in the nonaqueous solvent. Particles entirely ofmetal may be chosen from any suitable metal that is obtainable in thecolloid size range, including metals from Groups I-A, I-B, II-A, II-B,III-B, IV-B, V-B, VI-B, VII-B, VIII, III-A, IV-A, and V-A of thePeriodic Table. In this connection, the colloid size range is consideredto include particles having a diameter of 1 to 1000 millimicrons, or 10to 10,000 angstrom units. A preferred size range is 25 to 500, morepreferably 50 to 200, angstrom units. As the substrate, when coated withmetal, comprises the product, the latter may determine the choice ofsuch substrate characteristics as size, material, shape, etc.

Particles made partly of a nonmetal and partly of a metal may beobtained in accordance with the electroless deposition method describedand claimed in said copending application Ser. No. 647,344, filed June20, 1967, and as illustrated in Examples 2, 3 and 4 below. According tothis method, colloid size particles of material like alumina are broughtin contact with a sensitizing agent like stannous chloride to sensitizethe particle surfaces; the particles are removed and freed of excessagent and then contacted with an activating agent such as palladiumdichloride which forms palladium on the surfaces. Such palladiumfunctions as a catalyst for the next step, wherein the particles, afterremoval of excess activating agent, are mixed with a reducible salt of ametal of Groups I-B,

II-B, VI-B, and VIII and with a reducing agent therefor,

such as a mixture of nickel sulfate and sodium hypophosphite, thereby toreduce such salt to form free nickel and to deposit such nickel on theparticle surfaces. The resulting particles, which comprise a core ofalumina and an outer shell of nickel, may then be subjected to immersiondeposition as described herein. Besides alumina, other inorganic oxides,particularly refractory oxides, may serve as the substrate or core, suchas silica, silica-alumina, titania, thoria, zinc oxide, vanadia,chromia, zirconia, Inolybdena, etc.; also materials like carbon black,graphite, cellulose, glass, etc.; crystalline al-uminosilicate molecularsieves, natural and synthetic; polymers like the polyolefins,fluorinated hydrocarbon polymers, polyethers, polyamides, polysulfones,polystyrenes, vinyl polymers, acrylic polymers, and various natural andsynthetic resins. Particularly suitable are monodisperse particles likethose of rutile and polystyrene latexes, i.e., particles that are veryuniform, with the ratio of the diameters of the largest to the smallestparticles not being larger than about 2:1. Instead of using asensitizing agent like stannous chloride, it is feasible, as disclosedin the aforenoted copending application, to use a vaporous agent likehydrazine, thereby to avoid an excess. Furthermore, suitablemetal-coated particles may be produced according to a modification, alsodisclosed in said copending application, wherein the substrateparticles, after treatment with the activating agent such as thepalladium salt, are taken as the product; they thus comprise a core ofnonmetallic material having a thin shell of palladium or otheractivating metal.

The method of immersion deposition comprises dissolving the metalcompound in the solvent, adding one or more auxiliary agents of the typedescribed, and dispersing the substrate particles in the solution.Deposition of the metal of the metal compound on the substrate usuallybegins immediately, and at room temperatures, although highertemperatures, going up to refluxing, and even lower temperatures aresuitable. Times of deposition are variable, depending on the desiredthickness of deposit; thus they may extend for a minute or two to 1, 2,or 24 hours or more. Concentrations of metal compound in the solutionare preferably saturation concentrations but it will be understood thatlower concentrations are useful, going down to 1%, or less, by weight.Concentrations of the auxiliary agents are usually only 1 to 5% of thatof the metal compound. The substrate surfaces should, of course, beclean, and it may be observed that an advantage of using nonaqueoussolutions is that a separate degreasing step is not generally requiredas the solvent can accomplish this operation during the deposition.

It will be understood that the action of depositing a metal on thesubstrate by immersion deposition involves a replacement of the metal ofthe metal compound by the metal of the substrate surfaces, provided ofcourse that the metal of the compound is below that of the substratesurfaces in the EMF series. In the immersion deposition solution apotential difference exists between the two metals which increases asthe metals are more and more removed from each other in the EMF series.The metal higher up in the series is frequently described as moreactive, or less noble, or anodic, while the metal lower in the series incharacterized as less active, or more noble, or cathodic.

At the end of the deposition, the coated particles'are separated fromthe solution, as by centrifugation or filtration through a fine filter,washed and dried, and taken as product. It has a strongly adheringdeposit of less active or more noble metal, the thickness of which mayvary from a monolayer or two, or even a partial monolayer, up to severalmicrons, depending on the time of contact of the substrate with thesolution, the temperature, the concentration of metal compound, and theporosity of the substrate particle surfaces. Generally, thicknessincreases with time of contact, increases with increasing temperatureand concentration, and increases with increasing porosity of thesubstrate. Adherence may be varied by use of different modifiers andsubstrates, and by varying the thickness, the adherence increasing asthe thickness decreases. The amount of deposit may range from less than0.001 to 5 or 10%, or even to 50%, preferably 0.001 to 1 or 2%, byweight of the coated particle. The product particles do not exhibit anysubstantial irreversible coagulation; especially is this the case whenmonodisperse substrate particles, like polystyrene, are used. The colorof the product particles varies, depending on the deposited metal. Itmay be added that the particles are conveniently handled in the form ofsuspensions.

As described, the product is particularly useful as a catalyst, it beingunderstood that the deposited metal is one known to be catalytic for adesired reaction. This property is enhanced by the form of the product,i.e., by the fact that the metal may be in the form of very thin films,including monolayers, and partial monolayers, and because of theextremely large surface area which the product particles may exhibit.Where the product particles have a core of nonmetallic material,particularly a refractory inorganic oxide, it is pertinent to note thatthe surfaces of this core, by virtue of using a nonaqueous i-mmersiondeposition solution, are not affected as they might be in the presenceof aqueous solutions; thus with aqueous solutions such surfaces aremodified by hydrolysis, hydration, and/or ion adsorption, withconsequent adverse effects on a metal deposited thereover, particularlyon its catalytic activity, adhesion, and the like.

The factors of thin films and high surface area are also of significancein other applications like fuel cell electrodes, filter material,deposition material for photographic films, paint pigments, inks,conducting plastics and magnetic microstructures for magnetic tapes ordrums or discs.

Metal alloys may be deposited on a substrate by using two or moresuitable metal compounds in the immersion deposition solution; forexample, by using a mixture of platinum chloride and palladium chloride,an alloy of platinum and palladium may be deposited; a mixture of silverperchlorate and palladium chloride gives an alloy of silver andpalladium; a mixture of stannic chloride and silver perchlorate gives analloy of tin and silver; a mixture of stannic chloride and a zincdithiophosphate gives an alloy of tin and zinc; a mixture of silverperchlorate, platinum chloride, and palladium chloride gives an alloy ofsilver, platinum, and palladium.

If desired, the outermost metal layer of the product may be coated overby immersion deposition, using a difierent metal compound in thedeposition solution than the previous compound; or such product may besubjected to electroless deposition, followed or not by immersiondeposition. In other words, the product is reusable in the inventionlike other substrates.

The invention is illustrated by the following examples.

EXAMPLE 1 Colloidal copper powder in an amount of 0.2 g. was dispersedin 15 ml. benzene that had been previously dried over a S-angstromcrystalline aluminosilicate molecular sieve. Then 0.2 g. silverperchlorate was added, with agitation. The copper powder turned black,indicating a deposition of silver, in about 3 minutes.

EXAMPLE 2 Platinum was deposited on colloidal carbon black usingelectroless deposition followed by immersion deposition. First, 10 g.carbon black of 125 sq. m./ g. surface area was sensitized by treatmentwith 1 liter of an aqueous solution containing g. stannous chloride and40 ml. concentrated HCl. The resulting suspension was filtered through a0.45 micron millipore membrane filter and then washed carefully, withfiltration between washings, with 3 liters distilled water. The wetparticles were dispersed in 1 liter of an aqueous solution containing0.5 m1. of a palladium dichloride solution and 1 ml. concentrated HCl,and after separation of the particles, they were washed in the mannerdescribed. The wet particles were next dispersed in 1 liter of anaqueous solution containing 5 g. copper sulfate, 7 g. caustic soda, 10g. of a 37% w./v. solution of formaldehyde, and 25 g. of sodiumpotassium tartrate (Rochelle salt). The particles were separated andcarefully washed with 4 liters distilled water followed by 500 ml.acetone and finally air dried. Then 1 g. of the resulting copper-coatedcarbon black particles was placed in 10 ml. benzene containing 0.4 ml.acetic acid and 0.4 ml. acetic acid anhydride and 0.1 g. platinumtetrachloride and left overnight. The particles were separated, washedwith distilled water, then with acetone, and dried. Based on emissionspectroscopic analysis, the particles had a metal content of 2% platinumand 0.8% copper.

EXAMPLE 3 Platinum was deposited on high surface area graphite particlesby using the procedure of Example 2 except that graphite of surface area235 sq. m./g., corresponding to a particle size of less than 130angstroms, was employed instead of carbon black. Analysis showed thepresence of 2.5% platinum and 0.7% copper by weight.

EXAMPLE 4 Platinum was deposited on high surface area alumina particlesusing the procedure of Example 2 except that colloidal alumina of 275sq. m./g. surface area was used instead of carbon black, and high speedcentrifugation instead of filtration. Analysis showed the presence of 1%platinum and 3% copper by weight on the particles.

EXAMPLE 5 Chemically pure mossy zinc was immersed in 10 ml. of benzenecontaining 0.2 ml. concentrated acetic acid, 0.2 ml. acetic acidanhydride, and enough platinum tetrachloride to saturate. A modifier,0.1 g. of triethylenediamine, was also present. A black stronglyadhering deposit of platinum was obtained.

EXAMPLE 6 Mossy zinc as in the preceding example was immersed in 10 ml.benzene containing 0.2 ml. acetic acid anhydride, 0.1 g.di-dodecylnaphthalene sulfonic acid, and 0.1 g. leaddiisocpropylphosphorodithioate. After 24 hours a strongly adhering leaddeposit formed on the zinc.

EXAMPLE 7 A small piece of sodium was cut under dry benzene to expose afresh surface and to remove any excess moisture. Then 0.2 g. of silverperchlorate was dissolved in dry benzene and added to thefirst-mentioned benzene. Deposition of silver on the sodium took placeimmediately.

The periodic table classifications used herein are based on thearrangement distributed by E. H. Sargent & Co. and further identified bythe legend Copyright 1962 Dyna-Slide Co.

In forming the solution for irrunersion deposition, the sequence ofaddition of the metal compound, solvent, and substrate is not material.

In the light of the foregoing description, the following is claimed.

1. Method for depositing a metal on high surface area colloidalsubstrate particles comprising dissolving a compound of said metal in anonaqueous anhydrous solvent in which said metal compound is soluble,adding to the resulting nonaqueous anhydrous solution said substrateparticles, said substrate particles having at least the outer surfacesthereof comprised of a reactive metal which is above said firstmentioned metal in the electromotive series, and thereby depositing saidfirst mentioned metal on said substrate particles.

2. Method of claim 1 wherein said first metal mentioned is a platinummetal.

3. Method of claim 1 wherein said substrate particles are comprisedentirely of said reactive metal.

4. Method of claim 1 wherein each substrate particle is comprised of acore of a nonmetal and an outer shell of said reactive metal.

5. Method of claim 4 wherein said particles are made by electrolesslydepositing on said core a shell of said reactive metal.

6. Method of claim 5 wherein said substrate particles are monodisperse.

7. Method of claim 1 wherein said reactive metal is an alkali metal.

8. Method of claim 1 wherein said reactive metal is an alkaline earthmetal.

9. Method of claim 1 wherein said reactive metal is selected from thegroup comprising titanium, zirconium, molybdenum, tungsten, germanium,lead, tin, hafnium, and vanadium.

10. Method for depositiing platinum on high surface area colloidalsubstrate particles comprising electrolessly depositing on the particlesa reactive metal which is above platinum in the electromotive series,then adding the particles to an anhydrous solution of a platinumcompound dissolved in a non-aqueous solvent, and thereby depositingplatinum on the particles.

11. Method of claim 10 wherein said electroless deposition comprises thesteps of sensitizing the substrate, then activating the same, thentreating the same with a reducible salt of a reactive metal, reducingsaid salt to form free reactive metal, and depositing said reactivemetal on the substrate.

12. Method of claim 11 wherein said sensitizing step is carried out witha vaporous sensitizing agent.

13. Method for depositing a metal on high surface area colloidalsubstrate particles comprising electrolessly depositing on the particlesa reactive metal which is above said first mentioned metal in theelectromotive series, then bringing the particles into contact with asolution of a compound of said first mentioned metal dissolved in anonaqueous anhydrous solvent, and thereby depositing said firstmentioned metal on the particles.

14. Method of claim 13 wherein said substrate particles aremonodisperse.

15. Method of 'claim 14 wherein said substrate particles are comprisedof a nonmetal.

16. Method of claim 15 wherein said substrate particles are comprised ofcarbon black.

17. Method of claim 16 wherein said reactive metal is copper.

18. Method of claim 17 wherein said first mentioned metal is platinum.

19. Method of claim 15 wherein said substrate particles are comprised ofgraphite.

20. Method of claim 19 wherein said reactive metal is copper.

21. Method of claim 20 wherein said first mentioned metal is platinum.

22. Method of claim 15 wherein said substrate particles are comprised ofalumina.

23. Method of claim 22 wherein said reactive metal is copper.

References Cited FOREIGN PATENTS 5/1965 Great Britain. 9/ 1965 GreatBritain.

WILLIAM D. MARTIN, Primary Examiner M. R. P. PERRONE, JR., AssistantExaminer US. Cl. X.R.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,511, 683 Dat d May 12, 1970 Inventor(s) Wilton F. Espenscheid and IsraelJ. Heilweil It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line t, "Heilwell" should read --Heilweil--. Column 2, line35, "dithiophoesphates" should read --dithiophosphates--. Column 4, line17, "in" should read -is--. Column 5, line 7U,"diisoqropylphosphorodithioate" should read--diisopropylphosphorodithioate--. Column 6, line 27 (claim 2), 'firstmetal mentioned" should read --first mentioned metal--; line 47 (claim10), "depositiing" should read --depositing--; line 72 (claim 15),Method of claim 1 4" should read -Method of claim l3-. Column 8, thefollowing References should be cited:

2,599,978 6/52 Davis et a1 117-1ooX 2,898,228 8/59 Kelley 117 17 3,118,072 9/6 1 West at al 117-13oX 3,216,8 5 11/65 Brown 117-16oX3,228,881 l/66 Thomas ll7-l6lX 3,255,033 6/66 Schmeckenbecher ll7-7lX3,3 5,327 2/67 Schmeckenbecher llT-YlX 3,367,792 2/68 Levine 117- 17 3,1oo,o12 9/68 Golben 117-109 3, 11 1, +27 12/68 Levy 117-130 3, 12o,68o1/69 Gulla 117-71 89l, L9 L 3/62 Great Britain SIGNED AND WILLIAM E-BGHUYIM JR. Ed Fletcher, In Gomissioner of Patents Attesting Officer

